You can purchase a ZBrush for iPad subscription as per the instructions provided in the Subscription Upgrade section.
The following information covers steps to activate your license after purchasing a subscription.
After your subscription upgrade is complete, license activation occurs in the following cases:
1. You have subscribed to Maxon One or a stand-alone ZBrush subscription with your My Maxon account through the Plans & Pricing page, then you logged into ZBrush for iPad with the same My Maxon ID.
2. You purchased a ZBrush for iPad subscription from within the Apple Subscription section of Account Settings. After purchasing, ZBrush automatically activates your license and loads the licensed version of ZBrush for iPad.
In the above cases, if the license does not automatically activate when loading ZBrush:
a.) From the ZBrush Home screen, tap the Activate prompt at top of the screen.
b.) Open Account Settings and tap Activate License.